In the field of microwave electronics, a so-called Gilbert cell is often used as a mixer. The Gilbert cell was invented by Barrie Gilbert, and its basics are for example described in U.S. Pat. No. 4,156,283. It is also possible to use a Gilbert cell as an attenuator with a possibility to attenuate an RF signal all the way to zero, by applying a DC voltage on the LO input of the mixer.
A Gilbert cell P1 according to prior art will now be described with reference to FIG. 1. There is a first transistor Q1, a second transistor Q2, a third transistor Q3 and a fourth transistor Q4. There is a first input P2 for an input RF current iin, and a second input P3 for an RF current −iin. There is also a first output P4 for an output RF current iout, and a second output P5 for an RF current −iout.
A bias voltage source Vctrl provides a positive bias voltage which is applied to the bases of the first transistor Q1 and the fourth transistor Q4, and a negative bias voltage which is applied to the bases of the second transistor Q2 and the third transistor Q3, the bias voltages giving rise to bias currents. There is a corresponding DC bias current iQ1, iQ2, iQ3, iQ4, for each one of the transistors Q1, Q2, Q3, Q4, where each DC bias current iQ1, iQ2, iQ3, iQ4 affects the RF current through its corresponding transistor Q1, Q2, Q3, Q4. The resistance into the emitter of a transistor is inversely proportional to the magnitude of the bias current, and using the fundamental current laws, the RF current iRFQ1 passing through Q1 can be written as.
                              i                      RFQ            ⁢                                                  ⁢            1                          =                              i                          i              ⁢                                                          ⁢              n                                ·                                                    i                                  Q                  ⁢                                                                          ⁢                  1                                                                              i                                      Q                    ⁢                                                                                  ⁢                    1                                                  +                                  i                                      Q                    ⁢                                                                                  ⁢                    3                                                                        .                                              (        1        )            
There is a first branch from the first output P4 to the third transistor Q3 and a second branch from the second output P5 to the second transistor Q2. The RF current in the first branch, i.e. the RF current iRFQ3 passing through the third transistor Q3 can be written as.
                              i                      RFQ            ⁢                                                  ⁢            3                          =                              i                          i              ⁢                                                          ⁢              n                                ·                                                    i                                  Q                  ⁢                                                                          ⁢                  3                                                                              i                                      Q                    ⁢                                                                                  ⁢                    1                                                  +                                  i                                      Q                    ⁢                                                                                  ⁢                    3                                                                        .                                              (        2        )            
A Gilbert cell works by controlling the route for the input RF current through the transistors. The first output RF current iout at the first output P4 may be written as iout=iRFQ1−iRFQ3, which is summarized in the equation below:
                              i          out                =                                                            i                                  i                  ⁢                                                                          ⁢                  n                                            ·                                                i                                      Q                    ⁢                                                                                  ⁢                    1                                                                                        i                                          Q                      ⁢                                                                                          ⁢                      1                                                        +                                      i                                          Q                      ⁢                                                                                          ⁢                      3                                                                                            -                                          i                                  i                  ⁢                                                                          ⁢                  n                                            ·                                                i                                      Q                    ⁢                                                                                  ⁢                    3                                                                                        i                                          Q                      ⁢                                                                                          ⁢                      1                                                        +                                      i                                          Q                      ⁢                                                                                          ⁢                      3                                                                                                    =                                    i                              i                ⁢                                                                  ⁢                n                                      ·                                                                                i                                          Q                      ⁢                                                                                          ⁢                      1                                                        -                                      i                                          Q                      ⁢                                                                                          ⁢                      3                                                                                                            i                                          Q                      ⁢                                                                                          ⁢                      1                                                        +                                      i                                          Q                      ⁢                                                                                          ⁢                      3                                                                                  .                                                          (        3        )            
In the same way, a corresponding expression may be obtained for the second output RF current −iout at the second output P5.
As evident from the expression (3), there is a problem with this type of attenuator; the higher attenuation that is desired, the smaller the difference between the bias currents has to be. For example, 20 dB attenuation means that 45% of the bias current shall pass through Q3 and 55% of the bias current shall pass through Q1. Furthermore, 40 dB attenuation means that 49.5% of the bias current shall pass through Q3 and 50.5% of the bias current shall pass through Q1. In order to obtain these very small current differences, a very accurate control of the bias voltage is required due to the transistors' high transconductance when biased, i.e. a small variation of the base emitter voltage, less than a millivolt, causes the difference in the bias currents to be to large. At the same time, the circuit is extremely sensitive to resistive feedback at the emitters, since this affects the bias difference between the two transistors involved.
There is thus a need for a design of a similar attenuator which does not require such an accurate control of the bias currents.